1. Field of the Invention
Embodiments of the present invention relate generally to video processing and more specifically to edge-preserving vertical interpolation.
2. Description of the Related Art
Video frames are typically encoded in an interlaced format comprising a first field and a second field, each field having alternating lines of the video frame and each field being temporally separated. Video images are typically encoded and transmitted in such an interlaced format as a compromise between bandwidth and video image resolution. Since interlaced video frames are displayed using only half the lines of a full video frame, less system bandwidth is required to process and display these types of video frames. However, since the human eye typically cannot resolve a single video field, but rather, blends the first field and the second field, the perceived image has the vertical resolution of both fields combined.
Some types of video displays, such as progressive displays, require the use of de-interlaced video frames instead of interlaced video frames. For such displays, the video frames encoded in an interlaced format must be de-interlaced prior to display. There are several well-known methods to construct de-interlaced video frames. One such method is commonly referred to as the “bob” method in which a de-interlaced video frame is constructed from a single video field that is vertically interpolated. One drawback of this method is that objects within a scene that have diagonal or round edges are often incorrectly interpolated. This is due to the fact that vertical interpolation generally does not try to preserve edge information, but rather simply interpolates between a line above and a line below the missing line in the video frame. The resulting interpolated diagonal edges or round objects appear as stair steps, as opposed to smooth edges, and, thus, tend to decrease picture quality.
Several well-known techniques attempt correct this drawback in the bob method by trying to preserve edge information. One such approach attempts to determine the edge of an object by correlating video data within a 3×3 pixel window (other versions of this approach may use other sized pixel windows, such as a 5×3 or 7×3 pixel window). Specifically, pixel data from the top line of the window is correlated with pixel data from the bottom line in order to determine a horizontal shift between the top and bottom lines. The horizontal shift is assumed to coincide with the edge. Upon determining the edge of an object within the pixel window, pixels for the missing field may be constructed by interpolating along the determined edge. This approach, however, is relatively sensitive to “noisy” video data that has relatively greater amounts of intensity transitions due to either picture content or encoding errors. As is well-known, a video field with noisy data may produce false edges that, in turn, cause erroneous interpolated pixels to be produced. Further, this approach often fails to find edges when there are no obvious correlations among the pixels within the pixel window.
As the foregoing illustrates, what is needed in the art is a vertical interpolation technique that better accounts for edge information, thereby, increasing overall picture quality.